Harvest index, ratio of grain to total above ground biomass, has remained nearly constant around 50% in maize over the past 100 years (Sinclair, (1998) Crop Science 38:638-643; Tollenaar and Wu, (1999) “Crop Science 39:1597-1604). Thus, the quadrupling of grain yield over the last 50-60 years has resulted from an increase in total biomass production per unit land area, which has been accomplished by increased planting density (Duvick and Cassman, (1999) Crop Science 39:1622-1630). Selection for higher grain yield under increasing planting densities has led to a significant architectural change in plant structure that of relatively erect and narrow leaves to minimize shading. An undesirable consequence of higher density planting (or higher plant populations) has been the increased frequency of stalk and root lodging. The relationship between planting density and biomass production deviates significantly from linearity as the optimal density is approached for maximal biomass yield per unit land area. This is reflected in a proportionately greater reduction in the individual plant biomass, which manifests in the form of weaker stalks and hence increased lodging. In addition, approximately 20% of total biomass at maturity stays in the form of roots in the soil, contributing to its organic matter content (Amos and Walters, 2006). Since both stalk and root lodging are agronomic characteristics affecting harvest index, dwarf type plants could have potential advantages in yield stability.
Dwarf plants have had a major impact on agriculture. Dwarf varieties of wheat (and other small grain cereals) are widely used in North America due to both reduced potential for lodging and response to more intensive management and yield stability and potentially higher yields. There are other benefits that may be realized from the higher harvest index of dwarf crop plants including reductions in the amounts of pesticides and fertilizers required, higher planting densities and reduced labor costs. Dwarf plants provide ease in harvesting, simplified management of crops and potential reductions in water and nutrient use.
In view of the current trends of both increasing human population and the decreasing land area suitable for agriculture, increasing agricultural productivity is, and will continue to be, a challenge of paramount importance. Dwarf crop plants are important components of our agricultural production system. Increased usage of dwarf crop plants may help to meet the agricultural production demands of the future.
Genes that increase stalk strength, i.e., Cellulose Synthase, are responsible for cellulose production in crop plants, can be modified to increase size and strength of various plant tissues. Cellulose in a unit length of the maize stalk was found to be the best indicator of mechanical strength (Appenzeller, et al., (2004) Cellulose 11:287-299; Ching, et al., (2006)). Increasing cellulose concentration in the stalk dry matter could lead to improving stalk mechanical strength and increasing biomass which in turn increases yield and potentially harvest index. Improvements in plant strength (biomass) and growth of specific plant tissues (organs) provides plants with greater biomass and increased harvest index.
Flowering time determines maturity, an important agronomic trait. Genes that control the transition from vegetative to reproductive growth are essential for manipulation of flowering time. In maize, flowering genes provide opportunities for enhanced crop yield, adaptation of germplasm to different climatic zones and synchronous flowering for hybrid seed production. The development of inbred lines having modified flowering facilitates the movement of elite germplasm across maturity zones. In addition, additional opportunities exist to increase the rate of grain fill and/or grain dry down to complement changes in the onset of flowering.
The combined controlled expression of plant architecture genes, flowering time genes and dwarfing gene components within transformed plants would not only increase the yield potential and harvest index of crop plants but would also improve the agronomic characteristics that simplify management practices and increase the adaptation of crop species into new geographic areas.
This invention provides means for altering the harvest index of crop plants by modulating the expression of transgenes using multiple stacked plant genes and dwarf gene components, thereby modulating plant architecture. A component of Dwarf gene D8, the dimerization domain (DD), a leucine-zipper dimerization domain (SEQ ID NO: 9) is overexpressed as a dominant negative transgene. The transgene/dimerization domain component stacks are provided in a single transformation vector unit and are used to modulate specific plant organs of a plant that can increase growth, yield and harvest index in plants. The expression in specific plant tissues, such as roots, ears or tassels can lead to elongation of the specific plant organs.
These stacked units could be used to enhance crop plant performance and value in several areas including: 1) plant standability (composed of stalk and root lodging), harvest index and yield potential; 2) modification of specific plant organ size; 3) plant dry matter as a feedstock for ethanol or for other renewable bioproducts and 4) silage.